1. Field of the Invention
The present invention generally relates to image forming apparatuses for use in copiers, facsimiles, printers, and the like, and more particularly to a xerographic image processing apparatus having an image fixing apparatus for heating and fixing a toner image formed on a recording material.
2. Description of the Related Art
Most of the conventional fixing apparatuses for a xerographic image processing apparatus are of a heat roller type. The heat roller fixing apparatus includes a heating roller maintained at a predetermined temperature and a pressing roller having an elastic layer and press-contacted to the heating roller. A transfer material, for example paper which has an unfixed toner image thereon, is passed through a nip between the heat roller and the pressing roller and heated to fix the image. In this type of image fixing apparatus, there is a problem due to the so-called toner offset phenomenon. That is, toner undesirably transfers to the heating roller. In order to prevent the toner offset phenomenon, the temperature of the heating roller has to be maintained at an optimum level. This requires a large thermal capacity for the heating roller or a heating material. The large thermal capacity necessitates a longer period of time to increase the temperature of the heating roller to the predetermined level, thereby requiring a longer waiting time upon start of the apparatus.
In order to prevent the above mentioned problems, the following types of fixing apparatus have been proposed:
1) A fixing apparatus heating intensively a transfer material through a fixing film (Japanese Laid-Open patent Application No. 63-313182), PA1 2) A fixing apparatus having an electrically conductive, self-heating (resistance) film as the fixing film (called a conductive fixing film for short hereinafter), and heating a transfer material (Japanese Laid-Open Patent Application No. 3-144676).
FIG. 12 shows a conventional fixing apparatus having said conductive fixing film for use in a xerographic image processing apparatus.
Referring to FIG. 12, the fixing apparatus comprises a pair of electrodes 1d, a support element la supporting the electrodes 1d, a conductive fixing film 1b moving while in contact with a surface of the support element, and a pressing roller 1c located just opposite to the support element 1a. A transfer material 1h having an unfixed toner image 1e thereon and the conductive fixing film 1b are passed through a nip between the support element 1a and the pressing roller 1c.
There are two ways to move the conductive fixing film 1b. One way is shown in FIG. 13. Referring to FIG. 13, this type of fixing device employs an endless belt as the conductive fixing film. The film is driven by a driving roller 1f and is tensioned by the driving roller and a follower roller 1g. The other type is not shown in the Figures. This type (called non-endless belt type) of a fixing device employs a film take-up shaft. The film that has been wound on a feeding shaft is transferred or wound up on the film take-up shaft. Neither type of fixing device has been commercialized yet.
In the fixing device that employs the conductive fixing film, a long period of time is not required to increase the temperature of the heating element to the predetermined level. On the other hand, the temperature of the film as a heating element that directly contacts the transfer material cannot be constantly controlled. Because the film itself is moving during fixing, the temperature of the film itself cannot be detected. Consequently, the temperature of the conductive fixing film sometimes becomes too high, so that the electricity consumption is much greater, and which reduces the durability of the conductive fixing film.